Another compressor wiring question

[quik1500]

Ok, so I did a search and only found people wanting to plug their new compressor in their dryer outlet, but here is my question. I just bought a new compressor to put in my "work in progress" detached garage. My breaker box acts as a sub panel with separate ground and neutrals. The main breaker is at the house. I have a dedicated 20a breaker with awg 10/3 running about 20ft to a small room for my compressor to go in. I have a metal box with a pigtail, and the 2 powers, neutral, and ground. I want to use an outlet instead of hardwiring it.

My builder who helped me wire it, says to run a 4 prong and tie the neutral and grounds together at the compressor, Another guy said to use a 3 prong and tie the green pigtail into the bare ground, and cap the neutral in the box.

Will both ways work? Opinions? Should I replace my 20a breaker with a 30a?

Here's some quick pictures for reference

 

[pattenp]

Do not connect the neutral and ground together. Just cap off the neutral and leave it unused. Also depending on the HP rating of the motor on the compressor it may need to be hardwired to be NEC compliant. Just looked at pics, you need to hardwire it.

Edit: The #10 is romex so it's undersized. You need 35A, so #8 NM or #10 THHN in pipe is needed for 5HP. This is based on NEC requirements. Non code compliant wise to run the compressor you can use a 30A breaker on the #10 NM and most likely not have any problems. If startups of the compressor trip the 30A breaker you can upsize the breaker but you do need to hardwire and not use a plug.

 

[wyliesdiesels]

I cant stand it when people give the poor advice of bonding neutrals and grounds at any place other than a main service panel or 3-wire fed subpanel!

Yes it needs to be hardwired and 35a rated wire needs to be used to be code compliant.

 

[CNGsaves]

^ ^ ^ How about those "builders" . . . .F'in up a guys compressor in new house !!

 

[pattenp]

That builder needs to stay away from giving electrical help.

 

[quik1500]

Thanks for the help guys,I appreciate the help. I have very little understanding of house wiring. What would happen if the neutral and ground were tied together at the compressor? A friend of mine has the same compressor and he used the neutral for the ground, but they are all tied together in his panel.

 

[grizz_660]

FLA is 21.5 amps. Therefore, it can be wired to #10 wire, on a 30A breaker. #10 is good for 30Amps, and legally, you are allowed to load a breaker to 80% of its rating ( 24 amps in this case ). Since FLA ( full load amperage ) is 21.5, you are under the threshold.

Dont use a dryer outlet, just go to home depot and get a 30A / 250v receptacle. You may also need a 30A / 250v male cord end for your compressor ( depending on if there is a cord or not ).

 

[grizz_660]

If you used a 4 wire cable ( L1, L2, G, N ) to feed your compressor, and tied your neautral and grd together, then plugged it in and used it, nothing would happen. Your ground is installed as a reference point for testing, and a safety precaution for the equipment and user. So if one of the lines went to ground, the breaker or fuse would trip / blow. Your neautral wire goes to ground. You will get continuity between ground and neautral.

 

[JJThrasher]

I know its probably not correct by the book, however 17 year old me put a plug on my compressor when I wired it up. Its a 220v 3.7HP motor. I've never had an issue with it.

 

[grizz_660]

More important than the HP rating, is the FLA on the motor nameplate. When a motor or any piece of equipment is installed electrically, this is where we take the information from. A "5hp motor" is only one piece of information.

 

[CNGsaves]

More important than the HP rating, is the FLA on the motor nameplate. When a motor or any piece of equipment is installed electrically, this is where we take the information from. A "5hp motor" is only one piece of information.

Well you have NOT read very well all the prior posts by GJ Sparkies . . . .
. . . . . . AS . . . . . . HP most certainly IS the determination . .
of wiring and breaker requirement for compressors hard-wired.

You do NOT simply go by the FLA on the motor plate.

Sparky will soon respond with your spanking !!

 

[grizz_660]

FLA is the calculations of the motor and serfive factor already done, plus any other electrical components that may be involved.

746watts per hp
W / E = I
I x SF = fla

 

[justsam]

Actually FLA is not the issue, INRUSH current is.
At start up the first half cycle may be 20 times FLA. After that it is 4 to 8 times FLA for several seconds as the motor comes up to speed.

Motors cannot be treated as pure resistive loads, with contiuous current draws.

 

[md21722]

At stated above, NEC has different rules for motor loads. For motor loads, you take the motor nameplate HP and look up the FLC in the NEC tables... Per the table, a 230V 5HP single phase motor is 28A. Then multiple by 125% of FLC to determine wire size. That's 35A. 230V single phase motors (and compressors) do not need a neutral. They only use 2 hots and a ground.

 

[pattenp]

Quik1500 if you want to install your compressor in a non code compliant manner then listen to grizz_660. If you look at outlets and plugs with 30 amperage rating you'll also find that they have a HP rating of not much more than 3HP which is another reason it's not code compliant to use on a 5HP motor. As I said in my first post there is a non code compliant way and it will most likely work just fine and your house will not burn down. I'm just pointing out the correct way the compressor should be wired to meet electrical safety standards.

 

[wyliesdiesels]

FLA is 21.5 amps. Therefore, it can be wired to #10 wire, on a 30A breaker. #10 is good for 30Amps, and legally, you are allowed to load a breaker to 80% of its rating ( 24 amps in this case ). Since FLA ( full load amperage ) is 21.5, you are under the threshold.

Dont use a dryer outlet, just go to home depot and get a 30A / 250v receptacle. You may also need a 30A / 250v male cord end for your compressor ( depending on if there is a cord or not ).

Per the NEC, u dont wire motor circuits based on the FLA on the nameplate. U take the HP rating (5HP) and look up the FLC rating of a 5HP motor in the NEC FLC tables. (28a for 5HP)

Then the wire is to be sized at 125% of the FLC so in this case 35a rated wire. #10 NM-b has a MAX ampacity of 30a, so #8/2 NM-b would need to be used. Or u can use #10 THHN/THWN in pipe.

Breaker can be MAX 250% of 28a(NEC FLC)...

Also that 30a NEMA 6-30 plug and recepticle at home depot is most likely not rated for more than about 3HP so again, it will need to be hardwired!

Also, OP is your breaker panel within sight of and no more than 50' from the compressor? If not, then u will also need a disconnect at the compressor!

 

[grizz_660]

How do you think the FLA is calculated? Saying something is "5hp" is not enough imformation to wire it correctly.

746watts per HP, therefore 5 x 746 = 3730watts
3730 watts / 230v = 16.22amps
16.22amps x 125% ( service factor ) = 20.3 amps.

Their fla and my fla are slightly different, maybe due to rounding, dont know. But that is how you size your wire and breaker.

 

[grizz_660]

Motor table.

I do stand corrected

 

[pattenp]

How do you think the FLA is calculated? Saying something is "5hp" is not enough imformation to wire it correctly.

746watts per HP, therefore 5 x 746 = 3730watts
3730 watts / 230v = 16.22amps
16.22amps x 125% ( service factor ) = 20.3 amps.

Their fla and my fla are slightly different, maybe due to rounding, dont know. But that is how you size your wire and breaker.

Nobody is disputing your math, it is just not the correct way to wire a motor operated piece of equipment to be compliant with the NEC.

Edit: I see you found the motor table.

 

[justsam]

I might dispute the math a bit in that no motor is 100% efficient, more like 85% for this class, so make that 880 Watt/HP. I would also use 240VAC so may come out to a wash, with the higher voltage. Looks like it would come out to about 23Amps.

As discussed, and agreed, really need to use proper NEC tables.

 

[quik1500]

My breaker panel is about 20' from the compressor, on the same wall.

 

[grizz_660]

The motor table is to be used "as guides only" ( quoted from the book ). Of course I am using the CEC book, but this is how it reads.

28-704 (2) where the full load current rating is not marked, an equivalent full load current rating shall be determined from horsepower rating by referring to table 44 or 45, where applicable.

So, it is correct to say that a 5hp motor will draw 28A. However, as stated, that table is to be used a guideline when no nameplate is present. In the case of this compressor, we have all the information on the nameplate. Hp, A,sf. Its all there. So in this case, you would go by the nameplate, not the book.

 

[grizz_660]

If the NEC reads differently than the CEC, then thats fine. Im not trying to argue or fight witb anyone.

 

[CNGsaves]

^ ^ ^ If you're in CANADA, then UPDATE GJ Profile with that information.

Canada code may be different than applicable USA.

Sparkies like Diesel or PattenP can set you straight on NEC REQUIREMENTS in USA for hard-wired compressors. OP is in Ohio so USA rules apply.

 

[grizz_660]

Hence why I said, if the NEC reads differently. Maybe they can quote from the NEC, then we can all learn something

 

[jbjke]

Do you live in town or in the country? Do you have to pull permits and have it inspected? Some homeowners in states can do this and some states will not allow. You can wire it with anything you want that doesn't trip the breaker and "get by" but if a fault is found after a fire, the insurance company will go back to the person that installed it. I would think peace of mind would be to ask a local electrician to see what local code would be.

It may be smart to just hard wire with THHN in metal EMT since your only 20 feet. You will sleep better at night. You might need a few feet of seal tight for vibration at compressor. There should be local electricians that should help over the phone. Do not hook the green and white together though for insurance purposes of a new install ( compressor is new )


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[credleaf]

can anyone please tell me are better yet send me wireing diagram of a allen-bradley 509 magnetic switch for my compresor please

 

[pattenp]

can anyone please tell me are better yet send me wireing diagram of a allen-bradley 509 magnetic switch for my compresor please

You should start your own thread and not hijack this one.

 

[kingchevy]

Per the NEC, u dont wire motor circuits based on the FLA on the nameplate. U take the HP rating (5HP) and look up the FLC rating of a 5HP motor in the NEC FLC tables. (28a for 5HP)

Then the wire is to be sized at 125% of the FLC so in this case 35a rated wire. #10 NM-b has a MAX ampacity of 30a, so #8/2 NM-b would need to be used. Or u can use #10 THHN/THWN in pipe.

Breaker can be MAX 250% of 28a(NEC FLC)...

Also that 30a NEMA 6-30 plug and recepticle at home depot is most likely not rated for more than about 3HP so again, it will need to be hardwired!

Also, OP is your breaker panel within sight of and no more than 50' from the compressor? If not, then u will also need a disconnect at the compressor!

So if you must size the circuit based on HP rating instead of the nameplate FLA, what do you do when you have a "5HP spl" motor with 15 FLA? Are you forced to size that circuit at 35 amps?

 

[pattenp]

So if you must size the circuit based on HP rating instead of the nameplate FLA, what do you do when you have a "5HP spl" motor with 15 FLA? Are you forced to size that circuit at 35 amps?

If you fall under the NEC and the motor data plate list the HP then the HP is to be used to size the supply conductors using table 430.248. There is an exception for the FLA to be used instead of the HP when both are listed, but that only applies to motor operated appliances. I can't say I've seen a motor with SPL and HP both listed in the HP section of a data plate.

 

[grizz_660]

If you fall under the NEC and the motor data plate list the HP then the HP is to be used to size the supply conductors using table 430.248. There is an exception for the FLA to be used instead of the HP when both are listed, but that only applies to motor operated appliances. I can't say I've seen a motor with SPL and HP both listed in the HP section of a data plate.

This is the information I was hoping to see. Interesting how the same motor is to be wired different in the two countries.

 

[pattenp]

This is the information I was hoping to see. Interesting how the same motor is to be wired different in the two countries.

The NEC section for ampacity and motor rating is 430.6.

 

[quik1500]

If I am dead set on using an outlet is it ok to go to a local hardware store and buy a 50 amp range cord?

 

[pattenp]

If I am dead set on using an outlet is it ok to go to a local hardware store and buy a 50 amp range cord?

So are you changing the wire from #10 to #6 to have a 50A curcuit?

 

[Pooch897]

I have a similar question. This is the motor tag. I will have about. 30' run to the compressor. I have a brand new roll of 10-3 wire and 30 amp Breaker and was hoping to use it if possible. I know I only need 2 wires for this but figured to just leave the neutral wire capped. Will this work safely?


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[pattenp]

I have a similar question. This is the motor tag. I will have about. 30' run to the compressor. I have a brand new roll of 10-3 wire and 30 amp Breaker and was hoping to use it if possible. I know I only need 2 wires for this but figured to just leave the neutral wire capped. Will this work safely?

Yes you are fine with the supply conductors being #10 NM and using the 30A breaker. With no HP listed you use the 15 AMP rating which is less than 3HP by NEC. I assume there is an overload reset button on the motor.

 

[quik1500]

So are you changing the wire from #10 to #6 to have a 50A curcuit?

I was going to leave the 10 that's all ready in the wall alone, except add a 50a outlet. The only 3 prong 50a cord I could find was for an electric range. I figured a 50a outlet would be better than a 30a outlet.

 

[Pooch897]

It says 5hp


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[pattenp]

It says 5hp

The motor data label lists HP as SPL. If you are seeing 5HP on a tank label, that doesn't matter.

 

[pattenp]

I was going to leave the 10 that's all ready in the wall alone, except add a 50a outlet. The only 3 prong 50a cord I could find was for an electric range. I figured a 50a outlet would be better than a 30a outlet.

Not in this case. You can use a 6-30 plug and outlet and make up your cord using 10/3 SJOOW cord. If you also want to use the circuit for a welder that needs a least a 30a circuit then you can use a 6-50 outlet and plug because typically welders specify 6-50 plugs. Use what ever you want, 30A or 50A.